Sscr. XXXVII. STELLAR AND PLANETARY NEBULAE. 379 



Stellar nebulae form another class. These have a 

 round or oval shape, increasing in density toward the 

 center. Sometimes the matter is so rapidly condensed 

 as to give the whole the appearance of a star with a blur, 

 or like a candle shining through horn. In some in- 

 stances the central matter is so highly and suddenly 

 condensed, so vivid and sharply defined, that the nebula 

 might be taken for a bright star surrounded by a thin 

 atmosphere. Such are nebulous stars. The zodiacal 

 light, or lenticular-shaped atmosphere of the sun, which 

 may be seen extending beyond the orbits of Mercury 

 and Venus soon after sunset in the months of April and 

 May, is supposed to be a condensation of the ethereal 

 medium by his attractive force, and seems to place our 

 sun among the class of stellar nebulas. The stellar neb- 

 ulae and nebulous stars assume all degrees of ellipticity. 

 Not unfrequently they are long and narrow, like a 

 spindle-shaped ray, with a bright nucleus in the center 

 (N. 230). The last class mentioned by Sir John Her- 

 schel are the planetary nebulae. These bodies have 

 exactly the appearance of planets, with sensibly round 

 or oval discs, sometimes sharply terminated, at other 

 times hazy and ill-defined. Their surface, which is 

 blue or bluish white, is equable or slightly mottled, and 

 their light occasionally rivals that of the planets in vivid- 

 ness. They are generally attended by minute stars, 

 which give the idea of accompanying satellites. These 

 nebulae are of enormous dimensions. One of them near 

 v Aquarii has a sensible diameter of about 20", and 

 another presents a diameter of 12". Sir John Her- 

 schel has computed that, if these objects be as far from 

 us as the stars, their real magnitude, on the lowest esti- 

 mation, must be such as would fill the orbit of Uranus. 

 He concludes that, if they be solid bodies of a solar 

 nature, their intrinsic splendor must be greatly inferior 

 to that of the sun, because a circular portion of the sun's 

 disc, subtending an angle of 20", would give a light 

 equal to that of a hundred full moons; while on the 

 contrary, the objects in question are hardly, if at all, 

 visible to the naked eye. From the uniformity of 

 the discs of the planetary nebulae, and their want of 

 apparent condensation, he presumes that they may 



